This disclosure generally relates to investment casting, and more particularly, relates to silicone-based binders for use in forming the ceramic cores and shell molds employed in investment casting.
The manufacture of gas turbine components, such as turbine blades and nozzles, requires that the parts be manufactured with accurate dimensions having tight tolerances. Investment casting is a technique commonly employed for manufacturing these parts. The dimensional control of the casting is closely related to the dimensional control of a ceramic insert, known as the core, as well as the mold, also known as the shell. In this respect, it is important to be able to manufacture the core and shell to dimensional precision corresponding to the dimensions of the desired metal casting, e.g., turbine blade, nozzle, and the like.
In addition to requiring dimensional precision in the casting of the ceramic core, the production of various turbine components requires that the core not only be dimensionally precise but also be sufficiently strong to maintain its shape during the firing, wax encapsulation, and metal casting processes. In addition, the core must be sufficiently compliant to prevent mechanical rupture (hot tearing) of the casting during cooling and solidification. Further, the core materials generally must be able to withstand temperatures commonly employed for casting of superalloys that are used to manufacture the turbine components, e.g., temperatures generally in excess of 1,000° C. Finally, the core must be easily removed following the metal-casting process. The investment casting industry typically uses silica or silica-based ceramics due to their superior leachability in the presence of strong bases.
Investment casting cores made using low pressure casting techniques generally suffer from poor mechanical properties. In low pressure casting techniques, ceramic slurry containing a solvent and one or more binders is poured into a mold. Typical binders are sodium silicate, hydrolyzed ethyl silicate, or silica sol as described in U.S. Pat. No. 2,928,749. The slurry then “gels” resulting in a rigid solid (sometimes referred to as a “polymer solvent gel matrix”). The gelled component and solvent are then removed by heating and/or a combination of heating and solvent extraction. Poor mechanical properties resulting from the low pressure casting process causes difficulty in ejecting cast parts from the metal die following curing. To impart mechanical strength to the core, the alcohol solvent commonly employed in the low pressure casting process is ignited, bisque firing the part in the casting die prior to ejection. This firing step can lead to thermally induced flaws in the core, reducing its strength and increasing production scrap that continues throughout the metal casting process.
For example, the prior art includes the use of silica (cristobalite) or silica-zircon as core materials. Dimensional control of the silica core is difficult for at least two reasons. First, crystalline-based silica materials are susceptible to Martensitic-type phase changes during the casting process. Accordingly, as a practical matter, the degree of crystallinity prior to casting is closely controlled. Otherwise, the core may crack once it is cooled down while still in the associated mold. Secondly, thermal expansion differences between the silica core and the associated mold are typically very large. Accordingly, it is difficult, if not impossible, to tightly fix the silica core within an associated mold without rendering the silica core susceptible to cracking.
Aluminum oxide, or “alumina”, by itself, without a chemical or physical binder material, has also been identified as a potential core material, and is typically employed with reactive alloys. Unfortunately, cores comprised of alumina based ceramics are known to exhibit excessive thermal expansion and poor crush behavior. Such behavior is unacceptable for applications where dimensional precision is required during manufacture, such as in the production of directionally solidified metal eutectic alloys and superalloys, which are typically used for manufacturing of turbine components. Moreover, alumina cores are typically removed using an autoclave operation, which adds considerable expense to the process.
Further, shrinkage with a concomitant decrease in porosity results in a ceramic article with unsuitable mechanical properties for the casting of superalloys. In this regard, because there generally is a considerable thermal expansion mismatch between the ceramic and the alloy, hoop and longitudinal tensile stresses are experienced by the alloy upon cooling from the superalloy casting temperature. Accordingly, if the ceramic article is very dense (i.e., non-porous) with little plasticity and having a high resistance to deformation at elevated temperatures, this can lead to mechanical rupture or hot tearing of the alloy in the ceramic article.
Moreover, with regard to solvents, serious problems can sometimes occur. The various drying procedures available can result in shrinkage and warping of the article, as capillary forces draw the ceramic particles together. Green parts containing high levels of liquids often exhibit the most shrinkage. Moreover, parts that include both thin cross-sections and thicker cross-sections are very susceptible to cracking or distortion, as the thin sections dry faster than the thicker sections.
Investment casting molds, or shells, are similar to cores in that adherence to dimensional tolerances is required for quality castings. Unlike cores, investment casting shells are generally produced via layer-by-layer application over a pattern such that the shell cavity is defined by the shape of the pattern. Wax patterns are typically used due to the ease of fabrication and wax removal. The wax is removed by heating the shell to a temperature above its melting point and pouring out the wax. As a result, traditional manufacturing techniques such as slip casting or injection molding are difficult to implement in shell production.
As mentioned earlier, shells are generally produced using a layer-by-layer approach. In this approach, as described in U.S. Pat. No. 4,247,333, an alumina-based ceramic with a silica-based binder, similar to those listed for ceramic cores, is applied to the pattern surface, which is then coated with coarse alumina powder. A layer-by-layer process is employed to overcome the technical barriers associated with uniformly drying a bulk-ceramic article where the inner surface of the article is an impermeable wax interface. By applying relatively thin layers, drying uniformity is improved, and overall dimensional precision can be maintained. Consequently, the shell manufacturing step is a relatively lengthy process; and one in which mold thickness is largely defined by both composition and number of coatings.
As the designs of alloy castings become more complex, the performance of the mold becomes more critical. Consequently, techniques to strengthen the mold in critical locations have been employed, such as those outlined in U.S. Pat. No. 4,998,581. The need for strengthening arises from the fact that variable shell thickness is difficult to achieve using conventional layer-by-layer manufacturing techniques.
Accordingly, there remains a need in the art for improved ceramic slurries and more robust processes that provide cores and/or molds with the desired dimensional accuracy and mechanical properties with minimal shrinkage and warping.